This might seem counterintuitive if you’ve ever cooled yourself down by the stiff breeze of an electric fan, but a new review published in the Cochrane Library suggests that there’s no good evidence that fans help during a heatwave.

Unlike air conditioning, electric fans don’t actually cool the air, but bring in cooler air from outside if placed near a window. That backfires, however, when air temperatures rise over 95°F — using an electric fan when it’s that hot can actually increase your body’s heat stress by blowing air that is warmer than the ideal body temperature over your skin. You may still feel a cooling sensation as the fan’s breeze evaporates your sweat, but increases in hot-air circulation and sweat evaporation can actually speed heat-related illness, such as heat exhaustion.

“An increase [in] sweating can cause dehydration and electrolyte imbalances. If these fluids and electrolytes are not replaced quickly enough, there is a possibility [that fans] may do more harm than good,” say the authors of the review.

These health issues are particularly worrisome for high risk individuals, such as older people and babies, who are more vulnerable to extremes in temperature in part because they are less likely to recognize symptoms of excessive heat exposure. Older people are also more likely to have underlying medical conditions like heart disease that can be exacerbated by hot weather.

The new review paper has relevance for public-health experts who are working to keep people cool this summer — and beyond. It’s hot around the globe right now, and according to the Intergovernmental Panel on Climate Change, we can look forward to more heatwaves worldwide, thanks to global warming. Increasingly, countries are developing official plans for keeping their citizens from overheating, and many of these plans include recommending the use of electric fans. Same goes for organizers, restaurants and other establishments in London who are preparing for the Olympic Games.

“It is important to know about the potential benefits and harms of electric fans when choosing whether to use one. This is true if you are simply making a decision about your own use of a fan, but it also applies to broader public health decisions, such as whether to give electric fans to groups of people during a heatwave,” said review author Dr. Saurabh Gupta, a consultant in public health at Hertfordshire Community NHS Trust, in the U.K., in a statement.

For its part, the U.S. Centers for Disease Control and Prevention (CDC) recommends electric fan use only when temperatures are below the 90s. Otherwise, people should turn on the air conditioning to cool down. If you don’t have A/C, the CDC recommends taking a cool shower or bath, refraining from turning on the stove or oven to maintain a cooler temperature, and calling a local health department to see if there are any heat-relief shelters in your area.

The authors of the Cochrane paper based their findings on published and unpublished studies that compared people who used fans with those who didn’t during a heatwave. They also consulted health experts with keen knowledge of how heatwaves influence health. The researchers couldn’t find any gold-standard randomized trials examining the issue, but they did dig up some retrospective, observational studies on electric-fan use. The findings were highly varied: some concluded that when people used fans during a heatwave, they were less likely to suffer heat illnesses or heat-related death, but other studies found no such benefit.

Our review does not support or refute the use of electric fans during a heatwave and people making decisions about them should consider the current state of the evidence base. They might also wish to make themselves aware of local policy or guidelines when making a choice about whether or not to use or supply electric fans.

“This is an incredibly important area of research for people seeking relief and a strategy to prevent negative effects of heat, including hospitalization and death,” says Dr. Kay Dickersin, director of the U.S. Cochrane Center at Johns Hopkins University, who studies heatwaves but was not involved in the current review.